Persistent weak thermal stratification inhibits mixing in the epilimnion of north-temperate Lake Opeongo, Canada

Pernica, Patricia; Wells, Mathew G.; MacIntyre, Sally

doi:10.1007/s00027-013-0328-1

Cited by 32 publications

(31 citation statements)

References 61 publications

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“…As winds increased, the diurnal thermocline tilted downward, with the magnitude of the tilt and the extent to which heat was mixed downward depending on wind speed. Once the winds relaxed, the internal wavefield in the seasonal thermocline extended into the mixed layer, similar to observations in Pernica et al ().…”

Section: Resultssupporting

confidence: 87%

Effects of Wind and Buoyancy on Carbon Dioxide Distribution and Air‐Water Flux of a Stratified Temperate Lake

et al. 2018

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Improved calculations of emissions of greenhouse gases from stratified lakes require understanding the physical processes controlling transport of dissolved gases to the air‐water interface on diel, synoptic, and seasonal time scales. We address this issue during the transition from late summer to autumn cooling in a small temperate lake by combining micrometeorology, physical limnology, and carbon dioxide (CO2) measurements throughout the water column. Over the 26‐day campaign, the lake cooled and emitted CO2 with daily average loss of 23 mmol CO2 m−2 d−1. Over diel cycles, lake surface pCO2 decreased during daytime heating and increased during nighttime cooling, while daytime CO2 fluxes exceeded nighttime fluxes by 35% due to higher daytime wind speeds. We compared the effects of diel and synoptic weather patterns on the CO2 distribution within the lake and lake‐atmosphere CO2 flux. Increases in near‐surface pCO2 scaled with stratification and heat loss which moderated transport of dissolved gases into the mixed layer. When winds were above ~4 m s−1, lake‐scale circulations drove upwelling and downwelling that redistributed heat and carbon dioxide between the northeast and southwest basins. Short‐burst peak CO2 fluxes exceeded 50 mmol m−2 d−1 during windy periods associated with storms. However, the seasonal cooling‐induced transition to persistent deep mixing led to the highest CO2 concentrations in the mixed layer and at the surface and the highest sustained CO2 fluxes (approaching 100 mmol m−2 d−1).

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Section: Resultssupporting

confidence: 87%

Effects of Wind and Buoyancy on Carbon Dioxide Distribution and Air‐Water Flux of a Stratified Temperate Lake

et al. 2018

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“…Below a depth equal to |L MO | during heating, dissipation appeared to be somewhat enhanced, although a lack of data limits our confidence in this region of the water column. The enhancement may be due to persistence of turbulence due to nocturnal cooling (e.g., Figure 7e, day 273.3-273.35) or associated with increased shear from nonlinear waves in the thermocline which energize the water above [Pernica et al, 2014]. When winds and heat flux varied rapidly and when advection from processes such as differential cooling dominated shear production, the turbulence in the surface mixing layer was not in equilibrium with atmospheric forcing.…”

Section: Discussionmentioning

confidence: 99%

“…Depending on the strength of the diurnal stratification, the intensity of night time cooling and wind stress, z AML , may or may not reach the top of the seasonal thermocline. Limnological studies quantifying the diel variability of turbulence and thermal stratification within the upper water column include Imberger [1985], Jonas et al [2003], Anis and Singhal [2006], and Pernica et al [2014], and oceanographic studies include Shay and Gregg [1986], Brainerd and Gregg [1993], and Anis and Moum [1994].…”

Section: The Diurnal Cycle Of the Actively Mixing Layer And Of The Momentioning

confidence: 99%

Similarity scaling of turbulence in a temperate lake during fall cooling

et al. 2014

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Turbulence, quantified as the rate of dissipation of turbulent kinetic energy (e), was measured with 1400 temperature-gradient microstructure profiles obtained concurrently with time series measurements of temperature and current profiles, meteorology, and lake-atmosphere fluxes using eddy covariance in a 4 km 2 temperate lake during fall cooling. Ãw /kz, where u Ãw is the water friction velocity computed from wind shear stress, k is von Karman's constant, z is depth, and J B0 is surface buoyancy flux. Below a depth equal to |L MO | during cooling, dissipation was uniform with depth and controlled by buoyancy flux. Departures from similarity scaling enabled identification of additional processes that moderate near-surface turbulence including mixed layer deepening at the onset of cooling, high-frequency internal waves when the diurnal thermocline was adjacent to the air-water interface, and horizontal advection caused by differential cooling. The similarity scaling enables prediction of near-surface e as required for estimating the gas transfer coefficient using the surface renewal model and for understanding controls on scalar transport.

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“…Density gradients were determined by applying finite center differencing to the density profiles, which were calculated using the 0.1 m resolution temperature data and thermodynamic equations specific to freshwater conditions (Chen and Millero ). The top and bottom depths of the metalimnion were defined as the depths above and below the thermocline where N reached at least 35% of the value of N at the thermocline; these depths were found by moving away from the thermocline depth along the profile of N (Pernica et al ) and were verified with visual inspection. Finally, from the N profiles, we derived three additional metrics of thermal stratification: (1) the value of N at the thermocline; (2) the mean buoyancy frequency within the DCM (determined by averaging all N values from the density profiles that fell between the top and bottom depths of the DCM peak, i.e., μ ± σ , see above); and (3) the mean buoyancy frequency within the metalimnion (determined by averaging all N values from the density profiles that fell within the metalimnion).…”

Section: Methodsmentioning

confidence: 99%

Patterns and drivers of deep chlorophyll maxima structure in 100 lakes: The relative importance of light and thermal stratification

Leach

Beisner

Carey

et al. 2017

Limnology & Oceanography

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The vertical distribution of chlorophyll in stratified lakes and reservoirs frequently exhibits a maximum peak deep in the water column, referred to as the deep chlorophyll maximum (DCM). DCMs are ecologically important hot spots of primary production and nutrient cycling, and their location can determine vertical habitat gradients for primary consumers. Consequently, the drivers of DCM structure regulate many characteristics of aquatic food webs and biogeochemistry. Previous studies have identified light and thermal stratification as important drivers of summer DCM depth, but their relative importance across a broad range of lakes is not well resolved. We analyzed profiles of chlorophyll fluorescence, temperature, and light during summer stratification from 100 lakes in the Global Lake Ecological Observatory Network (GLEON) and quantified two characteristics of DCM structure: depth and thickness. While DCMs do form in oligotrophic lakes, we found that they can also form in eutrophic to dystrophic lakes. Using a random forest algorithm, we assessed the relative importance of variables associated with light attenuation vs. thermal stratification for predicting DCM structure in lakes that spanned broad gradients of morphometry and transparency. Our analyses revealed that light attenuation was a more important predictor of DCM depth than thermal stratification and that DCMs deepen with increasing lake clarity. DCM thickness was best predicted by lake size with larger lakes having thicker DCMs. Additionally, our analysis demonstrates that the relative importance of light and thermal stratification on DCM structure is not uniform across a diversity of lake types.

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Persistent weak thermal stratification inhibits mixing in the epilimnion of north-temperate Lake Opeongo, Canada

Cited by 32 publications

References 61 publications

Effects of Wind and Buoyancy on Carbon Dioxide Distribution and Air‐Water Flux of a Stratified Temperate Lake

Effects of Wind and Buoyancy on Carbon Dioxide Distribution and Air‐Water Flux of a Stratified Temperate Lake

Similarity scaling of turbulence in a temperate lake during fall cooling

Patterns and drivers of deep chlorophyll maxima structure in 100 lakes: The relative importance of light and thermal stratification

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